program Convex;

{$APPTYPE CONSOLE}

uses
  SysUtils;

var
  n, i, j, dS, neg, pos: Integer;
  mass1, mass2 : array of Integer;
  fin: TextFile;

begin
  AssignFile (fin, 'input.txt');
  Reset (fin);
  Readln (fin, n);
  SetLength (mass1, n+2);
  SetLength (mass2, n+2);
  for j:=0 to n do
  begin
    for i:= 0 to n do
    begin
      if i mod 2 = 0 then
      begin
        Read (fin, mass1[j]);
      end else begin
        Read (fin, mass2[j]);
      end;
    end;
  end;
  mass1[n+1]:=mass1[0];
  mass2[n+1]:=mass2[0];
  mass1[n+2]:=mass1[1];
  mass2[n+2]:=mass2[1];
  for i:= 0 to n+1 do
  begin
     dS:=(mass1[i]-mass1[i+2])*(mass2[i+1]-mass2[i+2])-(mass1[i+1]-mass1[i+2])*(mass2[0]-mass2[i+2]);
     if dS>0 then
     begin
       pos:=pos+1;
     end else begin
       neg:=neg+1;
     end;
  end;
  if (pos=n) or (neg=n) then
  begin
     Writeln ('yes.');
  end else begin
     Writeln ('no.');
  end;
  Readln;
  CloseFile (fin);
end.
